Timing chain choices, which is the right one?
 

I've heard different stories from different people and mechanics. Just wanted to see what advice I get here for my 1991 300d 602 engine with 210k miles.

1) Some people just check for chain stretch, adjust it, and that's it. Others say to replace the chain, rails, and tensioner. An article I just read said to replace the sprocket too.How many have replaced the sprocket?

2) A general mechanic is going to check the timing chain for stretch, or he can replace the chain for an additional $250. Does the chain need to be replaced?

3) One MB independent mechanic told me not to replace the chain. He said he has never seen a chain break on W124's; they are dual chains and they're NOT going to break. He also told me he checks for stretch through the injection pump, and that there was a service bulletin out that the injection pump also needs to be altered by 1 degree.

....So....what to do? Just check for stretch? Or replace parts? If so which ones? Is a new chain really necessary?

A new chain is only necessary if it is worn. Checking stretch is a reasonable method of checking (the chain doesn't actually stretch, it wears and the tolerances add up to a longer chain). The life of the chain has a lot to do with the driving style it has been subjected to and how well the engine has been maintained. If the oil change interval has been too long, used cheap oil, or drove aggressively all the time then the chain will wear faster.

The sprocket doesn't need to be replaced unless the teeth are worn. They're normally a square profile and fit fairly snugly in the spaces in the chain. If they're worn pointy or "sharp" get a new sprocket, it'll wreck the new chain. Unless the chain is really stretched and has been neglected for a LONG time, it's unlikely that the sprocket is bad.

For what it's worth, my SDL has the 6 cylinder 603 with more surely more miles than the Odometer says, and both the chain and sprocket in mine were still good. The cam timing is non-adjustable on the 60x engine. On the 61x you could use an offset woodruff key to "correct" the timing. The 60x uses a dowel pin to locate the sprocket to the cam and is not adjustable.

Do you think the rails and tensioner should be replaced?
 

What about rails and tensioner? The car has 210k on it. I'm thinking if the tensioner looks good.....the next time this car will be checked will be in another 200k and the tensioner could start to fail in 40k from now for all I know.

If someone has not already mentioned it only use a IWIS (looks like JIWIS on the chain).

It is not a good idea to let the Mechanic substitute another make of timing chain.

The tensioner doesn't have much to it. A couple springs, a piston, and a couple of ball bearings. There isn't really anything inside to go wrong except for varnish buildup. I tore down the tensioner in my SDL and cleaned it in solvent. Nothing was worn internally.

Rails are down to how worn they are. If they aren't worn out, leave them. If they're looking sad, replace them.

Don't know the 602, but in my 603 engines I used an offset key on the camshaft sprocket. Simple, and I have no concerns that the chain will ever break.

You sure about that? The FSM shows a locating dowel (no key) for the 603 and my SDL's engine mirrored that of the FSM.

...make sure you reset the tensioner properly...